An air-fuel ratio detection device applying a voltage to an air-fuel ratio sensor and detecting electric current that flows in the air-fuel sensor as a sensor current depending on an air-fuel ratio has been known. As this kind of air-fuel ratio detection device, JP 57-187646 A (corresponding to U.S. Pat. No. 4,419,190 A) discloses an air-fuel ratio detection device including a detection resistor connected in series with an air-fuel ratio sensor and applying a direct current (DC) voltage and an alternating current (AC) voltage to a series circuit of the air-fuel ratio sensor and the current detection resistor.
In the air-fuel ratio detection device disclosed in JP 57-187646 A, a voltage across the current detection resistor is amplified by an amplifier, and an output of the amplifier is inputted into a low-pass filter and a high-pass filter. Then, an output of the low-pass filter is measured as a sensor current, and a value obtained by holding peak of an output of the high-pass filter by a peak-value rectifier is used as a value depending on a temperature of the air-fuel sensor (also depending on an impedance of the air-fuel sensor) for controlling a heat resistor (heater).
In the above-described technique, a DC component corresponding to the sensor current is extracted from the output of the amplifier (the voltage across the current detection resistor) by the low-pass filter, and the sensor current (eventually, the air-fuel ratio) is detected from the extracted DC component. In the above-described configuration, the output of the amplifier is converted into direct current by the low-pass filter, and an output change of the low-pass filter is slower than a change of the sensor current associated with a change of the air-fuel ratio. Thus, a detection responsiveness of the sensor current (eventually, a detection responsiveness of the air-fuel ratio) becomes low.
In addition, in the above-described technique, an AC component of amplitude depending on the impedance of the air-fuel ratio sensor is extracted by the high-pass filter. Because the low-pass filter and the high-pass filter are respectively provided for detecting the sensor current and the impedance of the air-fuel sensor, a size of the device increases.